The present invention relates to a memory board forming a memory device in a data processing system and, more particularly, to a memory board in which, when a plurality of memory boards are inserted into a mother board, the selection of each memory board may be made with a simple circuit without any manual operation of assigning the address range of each memory board.
A memory device used in an electronic computer is generally constructed such that a plurality of memory boards of the same kind are inserted into a mother board with properly-wired contact terminals. For example, in the case of a memory device with a maximum memory capacity of 512 KB (kilobytes, K=1024), eight memory boards are required for the construction of the memory device when each memory board has a memory capacity for 64 KB. Similarly, two memory boards are necessary for the construction of a memory device when each memory board has a memory capacity of 128 KB; four memory boards for a memory device with a memory capacity of 256 KB.
For constructing memory devices with memory capacities of 256 KB, 128 KB and 64 KB by using a memory board with a maximum memory capacity of 512 KB, for example, a number of memory chips are mounted on the memory board corresponding to 1/2, 1/4 and 1/8, respectively, the number of those memory chips mounted on the memory board to achieve the 512 KB capacity. Accordingly, those memory devices have spaces where no memory chips are located on the memory boards, respectively.
Before use, the addresses of the memory board must be set. For example, in the case of the memory board of 64 KB, it is determined what addresses are assigned to the memory board; for exammple, 0 to (64K-1)B, 128 KB to (192K-1)B or 256 KB to (320K-1)B. For such an address assignment, two methods are known; one using base address switches for the respective memory boards and the other employing jumper wires. In the former method, the start address of a memory board is set by the base address switch associated with the memory board. Disadvantages of this method are the base address switches must be used and the start address must be set every time the memory board is inserted into the mother board. In the latter method, the start address of the memory board is set in a manner such that a memory address transferred from a central processing unit or an input/output control unit is decoded by a decoder and the output terminal of the decoder is coupled with the address line of the memory board by the jumper wire. The major disadvantage of this method is that jumper wiring must be made every time the memory board is inserted into the mother board.